Works with JSON-server at localhost:8080 for now (see src/lib/todoService.js)

This is a simple todo app from a very useful React course by Andy van Slaars. I made a few tweaks:

• Implemented error handling and UI updates for database request failures
• Experimented with abortable fetch for canceling async requests before unmounting
• Added AbortController and fetch polyfills for Jest
• Set up a "bad" JSON server that rejects all connections for testing
• Used the new version of React Context API
• Switched to react-scripts@next to load SVG icons inline via Webpack (tip source)
• Made the app fully functional for keyboard-only users
• Cleaned up unnecessary divs, improved HTML semantics and accessibility
• Added styles and icons

TODO: refactor CSS, turn footer into accessible <nav> and style it, add pagination on todo list (via json-server pagination).